The present invention relates to the formation of soot used in the manufacture of glass and, more particularly, to a method of delivering liquid precursors and other reactants into a flame to create soot for use in the manufacture of optical waveguides, and optical waveguides made by the method.
While the invention is subject to a wide range of glass soot applications, it is particularly well suited for the deposition of soot on a target to form preforms used in the manufacture of optical fibers, and will be particularly described in that connection.
Various processes are known in the art that involve the production of oxides, and particularly, metal oxides from vaporous reactants. Such processes require a feedstock solution or precursor, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site (also known as a soot reaction zone or combustion zone to those skilled in the art), and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected in any number of ways, ranging from capture in a collection chamber to deposition on a rotating mandrel. The collected soot may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles, injectors, burners and/or burner assemblies.
Much of the initial research that led to the development of such processes focused on the production of bulk silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of between 200-300 millimeters of mercury (mm Hg) at temperatures below approximately 100xc2x0 C. would be useful for making such bulk silica. The high vapor pressure of silicon tetrachloride (SiCl4) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other is responsible for the presently accepted use of SiCl4, GeCl4, POCl3, and BCl3 as feedstock vapor sources.
Use of these and other halide-based feedstocks as vapor sources, however, does have its drawbacks. The predominate drawback being the formation of hydrochloric acid (HCl) as a by-product of oxidation. HCl is not only detrimental to the deposition substrates and the reaction equipment, but to the environment as well. Overcoming this drawback, amongst others, led to the use of halide-free compounds as precursors or feedstocks for the production of soot for optical waveguides.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, for example, avoids the formation of HCl, other problems remain, particularly when the soot is intended for the formation of optical waveguides. It has been found that, in the course of delivering a vaporized polyalkylsiloxane to the burner, high molecular weight species can be deposited as gels in the lines carrying the vaporous reactants to the burner, or within the burner itself. This leads to a reduction in the deposition rate of the soot that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that often produce defective and/or unusable optical waveguide fiber from the effected portions of the blank. An additional problem encountered while forming silica soot using siloxane feedstocks is the deposition of particulates having high molecular weights and high boiling points on the optical waveguide fiber blank. The build-up of these particulates results in xe2x80x9cdefectxe2x80x9d or xe2x80x9cclustered defectxe2x80x9d imperfections that adversely affect the optical and structural quality of optical waveguides formed using the silica soot.
Other feedstocks, some of which are, and others of which may be useful in forming soot for the manufacture of optical waveguides are not currently acceptable alterative to the halide-based and halide-free feedstocks for delivery via vapor deposition. Materials such as salts and those known as rare-earth elements, for example, are extremely unstable as vapors and often decompose before they can be delivered in their vapor phase. Rather than being delivered from the burner as a vapor, these elements tend to form solids that plane out of the solution.
Although it is often possible to deliver at least a percentage of these elements to the combustion zone as a vapor, it is technically very difficult. Elaborate systems incorporating expensive equipment are necessary to convert these elements to the vapor phase, and further, to deliver them to the combustion zone without leaving behind deposits of solids in the lines leading to the burners and in the burners themselves. Moreover, if multiple elements are being delivered as vapors and a specific percentage of each is necessary for the desired composition, it is difficult to control the delivery to provide that percentage since different elements have different vapor pressures.
U.S. patent application Ser. No. 08/767,653, discloses that these and other limitations can be overcome by delivering a feedstock to an injector or burner in liquid form, atomizing the feedstock to form an aerosol containing fine droplets of the liquid feedstock, and converting the atomized liquid feedstock into soot at the combustion zone. The injectors, burners, and burner assemblies disclosed in U.S. patent application Ser. No. 08/767,653 rely on very small orifices to deliver the liquid in a fine stream for proper atomization. Because the feedstocks are delivered directly into the burner flame as liquids rather than vapors, the vapor pressures of the feedstocks are no longer limiting factors for delivery. Accordingly, many additional elements can now be delivered as feedstocks or dopants to form soot for use in the manufacture of optical waveguides.
A number of elements however, particularly those which are typically categorized as salts, are not easily delivered to a flame in liquid form as an organometallic compound. The purity requirements are often extremely high, as are the costs associated with attempting to obtain compounds of the required purity.
There is a need therefore, for a method of manufacturing soot for use in making optical waveguides, and particularly preforms for optical waveguide fibers that enables a user to precisely control the quantity of elements being delivered, and at the same time, eliminates gelling in the delivery lines. Further, what is needed is a liquid delivery method that produces glass soot containing metal oxides, traditional dopants, and salts in the required stoichiometry without requiring expensive and elaborate equipment.
The present invention is directed to a method for delivering liquids and other reactants to a combustion zone adjacent a burner assembly to produce soot for use in the manufacture of glass. In a liquid delivery system, a liquid reactant, capable of being converted by thermal oxidative decomposition to glass, is provided and introduced directly into the combustion zone of a combustion burner, thereby forming finely divided amorphous soot. Examples of such liquid delivery systems are disclosed in U.S. patent application Ser. No. 08/767,653, filed Dec. 17, 1996, and entitled xe2x80x9cMethod and Apparatus for Forming Fused Silica by Combustion of Liquid Reactantsxe2x80x9d; U.S. patent application Ser. No. 08/903,501, filed Jul. 30, 1997, and entitled xe2x80x9cMethod for Forming Silica by Combustion of Liquid Reactants Using Oxygenxe2x80x9d; U.S. patent application Ser. No. 09/089,869, filed Jun. 3, 1998, and entitled xe2x80x9cMethod and Apparatus for Forming Silica by Combustion of Liquid Reactants Using a Heaterxe2x80x9d; U.S. Provisional Application Serial No. 60/068,255, filed Dec. 19, 1997, entitled xe2x80x9cBurner and Method for Producing Metal Oxide Sootxe2x80x9d; and U.S. Provisional Application, filed Jul. 31, 1998, and entitled xe2x80x9cMethod and Apparatus for Forming Soot for the Manufacture of Glass,xe2x80x9d the specifications of which are hereby incorporated by reference. The amorphous soot can be captured in any number of ways, but is typically deposited on a receptor surface where, either substanually simultaneously with or subsequent to its deposition, the soot is consolidated into a body of fused glass. The body of glass may then be either used to make products directly from the fused body, or the fused body may be further treated, e.g., by forming an optical waveguide such as by drawing to make optical waveguide fiber as further described in, for example, U.S. patent application Ser. No. 08/574,961 entitled, xe2x80x9cMethod for Purifying polyalkylsiloxane and the Resulting Productsxe2x80x9d, the specification of which is hereby incorporated by reference.
The method of the present invention provides a number of advantages over other glass soot production methods known in the art. The present invention provides the capability of precisely varying and controlling the composition of the soot produced, which in turn provides for optical waveguide fibers having well defined and highly accurate index profiles, and other characteristics. The present invention further affords the industry with a method of concurrently delivering the greatest number of elements to a flame, to produce a multi-component glass soot. Any of a number of the organometallics, the rare earth elements, and now salts can all be concurrently delivered to a flame to produce homogenous soot. Similarly, these elements can be concurrently, or selectively delivered during the same soot production run to produce a preform meeting specific layering requirements. Accordingly, an optical fiber preform made by the method of the present invention has the advantage of containing precise quantities of elements, some of which have never been combined within a single optical waveguide fiber preform.
To achieve these and other advantages, a non-aqueous liquid reactant and an aqueous solution are atomized to form an aerosol made up of numerous liquid droplets. The aerosol is delivered into a combustion zone and reacted in the flame of the combustion zone to form finely divided glass soot particles.
In another aspect of the invention, a non-aqueous liquid reactant and an aqueous solution are delivered to a burner assembly. The non-aqueous liquid reactant and the aqueous solution are discharged from the burner assembly into a flame where they are reacted to form soot. The soot is deposited onto a target to form a preform.
In yet another aspect of the invention, an optical fiber preform is formed by the process of delivering a non-aqueous liquid reactant and an aqueous solution to a burner assembly. The non-aqueous liquid reactant and the aqueous solution are discharged from the burner assembly into a flame as an aerosol formed of a plurality of non-aqueous liquid reactant droplets and a plurality of liquid aqueous solution droplets. The droplets are reacted in the flame to produce soot and the soot is deposited on a target to form the preform.
In a further aspect of the invention, the delivery of an aqueous solution is combined with conventional vapor delivery. The aqueous solution is atomized with a gas at a burner assembly to form an aerosol made up of numerous liquid droplets, and another reactant is vaporized for delivery to the burner assembly. The vaporous reactant and the aerosol are reacted in a combustion zone adjacent the burner assembly to form finely divided glass soot.
Additional features and advantages of the invention will be set forth in the detailed description, which follows, and in part will be apparent from the description, or may be learned by practice of the invention. It will be understood by those skilled in the art that both the foregoing general description and the following detailed description are exemplary and explanatory in nature and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate several embodiments of the invention, and together with the description serve to explain the principles of the invention.